The present invention relates to a threaded coupling having a diameter in the interval of 30 to 40 mm adapted for percussion drill rods and comprising an external thread on the rod and an internal thread in the sleeve, the threads both of the rod and the sleeve having continuously curved tops and bottoms in the longitudinal section of said threads.
In drill steel couplings a type of rope thread called the R16-profile has been used for a long time. In equipment for drifter drilling it is totally dominating.
The R16-profile has a good fatigue resistance in combination with a satisfying wearability thanks to the rounded shape of tops and bottoms of the thread.
It has however turned out that the R16-profile does not function in a satisfying way together with the high frequency percussive drilling machines that have been developed during recent years.
The problem is that the thread coupling due to the high percussion frequency does not stay sufficiently tightened during work. This means that the degree of efficiency for the energy of the shock wave decreases and so-called pittings develop on the contact surfaces of the thread. These pittings do eventually lead to a fatigue fracture.
The aim of the invention is thus to prevent a thread coupling that remains sufficiently tightened during work without deteriorating the good characteristics in fatigue resistance and wearability.